A device based on a piezoelectric phenomenon has been used in a broad field. In the progress of miniaturization and power saving of portable telecommunication equipments such as a cellular phone, use of surface acoustic wave (SAW) devices such as filters for RF and IF has been enlarged. Advanced design and producing technologies of SAW filters have satisfied user's strict requirements to specifications. However, as frequencies for use are shifted to higher frequencies, the enhancement of characteristics has approached its upper limit, and great technical innovation has been required for both of microstructure of electrodes to be formed and maintenance of stable outputs.
On the other hand, a thin film bulk acoustic resonator (hereinafter referred to as FBAR) using thickness vibration of a piezoelectric thin film, and stacked thin film bulk acoustic wave resonators and filters (hereinafter referred to as SBAR) are constructed by forming a thin film mainly composed of piezoelectric material and electrodes for driving the film on a membrane disposed on a substrate, and fundamental resonance is possible in a giga-hertz band. Band-pass filter constituted by FBAR or SBAR can remarkably be miniaturized, and can also be operated with a low insertion loss and in a broad pass band. In addition, the band-pass filter can be manufactured integrally with a semiconductor integrated circuit, and is therefore expected to be applied to future ultra-miniature portable telecommunication equipments.
SAW filters, applied to the resonator, band-pass filter, and the like using the surface acoustic wave are manufactured by forming a interdigitated structure electrode or reflector for exciting the surface acoustic wave on the surface of a wafer consisting of piezoelectric single crystals such as lithium niobate and lithium tantalate. After that, the wafer is divided into pieces of device unit to obtain the SAW filters. In recent years, the following method for manufacturing the SAW filter has also been studied. A piezoelectric thin film is formed on a semiconductor single crystal substrate such as silicon, a hard substrate obtained by forming polycrystalline diamond on a silicon wafer, and the like by applying various thin film forming methods. Next, interdigitated structure electrode or reflector for exciting the surface acoustic wave is formed on the surface of the piezoelectric thin film by micro-fabrication or patterning technique. Finally, the substrate is divided into pieces of device unit to obtain the SAW filters.
Moreover, the thin film piezoelectric devices (hereinafter referred to also as the piezoelectric thin film devices) such as FBAR and SBAR applied to the resonators, band-pass filters, and the like using the acoustic wave are manufactured as follows. A base film including a dielectric thin film, a conductive thin film, or a stacked film composed of these is formed on a semiconductor single crystal substrate such as silicon, a substrate including polycrystalline diamond formed on the silicon wafer, or a substrate formed of metals with constant elastic modulus such as elinvar by various thin film forming methods. The piezoelectric thin film is formed on the base film, and an upper structure including a required constitution is further formed. After forming each layer or all layers, each film is subjected to micro-fabrication and patterning by a physical or chemical treatment. After preparing a bridge structure in which a portion positioned under a vibratory section of the piezoelectric thin film is removed from the substrate by anisotropic etching, the structure is finally divided into pieces of device unit to obtain a thin film bulk acoustic resonator (hereinafter referred to also as the thin film piezoelectric resonator).
For example, for the thin film piezoelectric device described in JP(A)-58-153412 or JP(A)-60-142607, after forming the base film, lower electrode, piezoelectric thin film, and upper electrode on the substrate, a via hole is formed to manufacture the device by removing a substrate portion positioned under a portion of the vibratory section from the back surface of the substrate.
Aluminum nitride (AlN), zinc oxide (ZnO), cadmium sulfide (CdS), lead titanate [PT] (PbTiO3), lead zirconate titanate [PZT] (Pb(Zr, Ti)O3), and the like are used as piezoelectric materials for the thin film piezoelectric device. Especially, AlN has a high propagation speed of the acoustic wave, and is suitable as the piezoelectric material for the thin film resonator or thin film band-pass filter which operates in a high frequency (microwave) band.
Various studies have heretofore been conducted so as to apply an AlN thin film to FBAR or SBAR. However, the thin film resonator or thin film band-pass filter which fulfils a sufficient performance in a giga-hertz band has not been obtained yet, and there has been a demand for improvement of an acoustic quality factor (Q value), temperature coefficient of frequency, and insertion loss of the AlN thin film. A thin film piezoelectric device superior in all of the acoustic quality factor (Q value), broad band operation, and frequency-temperature characteristic and indicating high performance in resonance characteristic has not been proposed. An electromechanical coupling coefficient is an important parameter which influences the above-described performance in constituting the resonator or band-pass filter, and largely depends on a crystal quality of the piezoelectric thin film for use.
To solve the problem, an object of the present invention is to provide a thin film bulk acoustic resonator in which characteristics of the AlN thin film having the high propagation speed of the acoustic wave are used, and which is large in the electromechanical coupling coefficient and superior in the acoustic quality factor (Q value) and frequency-temperature characteristic or which is superior in characteristic aspects such as a bandwidth, insertion loss, and gain, and which is remarkably high in characteristics and performance as compared with a conventional resonator.
Moreover, an object of the present invention is to provide a thin film piezoelectric resonator in which the characteristics of the AlN thin film are used, and which is large in the electromechanical coupling coefficient and therefore superior in the acoustic quality factor (Q value), bandwidth and frequency-temperature characteristic, and which can realize FBAR or SBAR with superior characteristics and high performance as compared with the conventional resonator. Furthermore, an object of the present invention is to provide a stacked structure which is suitable for realizing the thin film piezoelectric resonator comprising aluminum nitride thin film.